Typical optical-fiber amplifiers and lasers are apparatus that provide optically pumped gain useful for creating and sustaining laser oscillations and/or for providing gain to an optical signal. In some embodiments, such optical-fiber amplifiers and lasers employ, for example, optical fibers doped with one or more rare-earth species that absorb pump light/radiation at one or more first wavelengths (the pump wavelength(s)) and then provide stimulated emission at one or more second wavelengths (the gain wavelength(s)).
When an ion of the rare-earth species (such as erbium or ytterbium, for example), already excited to a higher energy level by absorption of a pump photon, interacts with a first photon having a wavelength within a narrow range specific to the type of ion and its ionization state, the ion undergoes stimulated emission wherein an electron of the ion drops to a lower energy state in the ion emitting energy equal to its drop in energy such that two photons result, wherein both resulting photons have the identical frequency, phase, direction, and polarization as the first photon. Such stimulated emission can be used for amplification of a light signal at the gain wavelengths, with applications including optical-fiber lasers and amplifiers.
Some conventional optical waveguides, such as the typical optical fibers well-known in the art, include a core surrounded by cladding having an index of refraction that is lower than the index of refraction of the core. Such conventional optical fibers typically maintain light in the core by total internal reflection. Other conventional optical waveguides, such as optical fibers, maintain light localization of light or control of electromagnetic fields in cavities or waveguides by applying a different physical property—the so-called photonic-band-gap (PBG) effect, as described, for example, in U.S. Pat. No. 6,845,204 issued 18 Jan. 2005 to Broeng et al., and U.S. Pat. No. 6,856,742 issued 15 Feb. 2005 to Broeng et al., which are both incorporated herein by reference. (Devices that exhibit the so-called PBG effect are also called photonic-crystal devices.)
In certain conventional optical-fiber lasers and amplifiers (and certain conventional optical waveguides formed at a surface of a planar substrate), pump radiation is introduced directly into the core. Other optical apparatus include cladding-pumped optical fibers, such as described, for example, in U.S. Pat. No. 6,625,363 issued 23 Sep. 2003 to Carter et al., which is incorporated herein by reference. Still other conventional apparatus include cladding-pumped waveguides that are fabricated on slab-type substrates such as described in U.S. Patent Application Publication Number 2002/0106150 A1 published 8 Aug. 2002, now U.S. Pat. No. 6,954,564 issued 11 Oct. 2005 to Bendett, which is incorporated herein by reference.
In an active or amplifying optical fiber, the core is typically used to carry and/or amplify light of a signal wavelength (e.g., a longer wavelength such as 1060 nanometers (nm)) by absorbing light of a pump wavelength (e.g., a shorter wavelength such as 960 nm), and providing energy for stimulated emission amplification (typically called lasing if done in a resonant cavity) of radiation of the signal wavelength. In many applications, there are one or more lowest-order modes (in free space, the lowest order mode has a Gaussian profile and is called TEM00, while in a fiber's core, the lowest order mode has generally linearly polarized Gaussian profile and is called LP01). The highest power of this lowest-order mode is typically in the center of the core, while higher-order modes tend to exist at the outer core or its boundary. When the lowest-order mode becomes dominant, it depletes energy from the pump-light excited lasing species in the center, while allowing a higher population of energized lasing species at the core boundary, which allows amplified spontaneous emission (ASE) and high-order modes (i.e., where the core is supporting multiple modes) to be amplified around the outer core, to the detriment of the dominant desired mode. Since at least some of the light of the ASE and/or the higher-order modes is at exactly the same wavelength as the desired signal, it is difficult to remove using wavelength-selective filtering. Because of quantum effects, the light traveling along the core boundary (even portions of the lowest-order mode) will form a so-called evanescent wave existing both just outside and just inside the core, and that light will leak from the core in certain circumstances, for example if the fiber is bent, or if the core has a diffraction grating imprinted on it. If that leaked signal light later re-enters the core, it will not be in phase with the desired dominant mode, and will constitute noise or undesired bandwidth spreading of the desired signal. Some fibers supply pump light in a cladding that surrounds a length of the fiber's core. It is undesirable to have signal light in that pump cladding, for the reasons just described.
Certain configurations of optical waveguides, including optical fibers, exhibit undesirable amplified spontaneous emission (ASE), higher-order modes, or other noise propagation or amplification in the cladding or external pump waveguide (typically called cladding modes). There is a need for improved optical devices including optical fibers that suppress cladding modes.